Observation of an Excess of Dicharmonium Events in the Four-Muon Final State with the ATLAS Detector

A search is made for potential ccc̄ c̄ tetraquarks decaying into a pair of charmonium states in the four ffiffi muon final state using proton-proton collision data at ps ¼ 13 TeV, corresponding to an integrated luminosity of 140 fb⁻¹ recorded by the ATLAS experiment at LHC. Two decay channels, J=ψ þ J=ψ → 4μ and J=ψ þ ψ ð2SÞ → 4μ, are studied. Backgrounds are estimated based on a hybrid approach involving Monte Carlo simulations and data-driven methods. Statistically significant excesses with respect to backgrounds dominated by the single parton scattering are seen in the di-J=ψ channel consistent with a narrow resonance at 6.9 GeV and a broader structure at lower mass. A statistically significant excess is also seen in the J=ψ þ ψ ð2SÞ channel. The fitted masses and decay widths of the structures are reported.